elf: Implement __libc_early_init

This function is defined in libc.so, and the dynamic loader calls
right after relocation has been finished, before any ELF constructors
or the preinit function is invoked.  It is also used in the static
build for initializing parts of the static libc.

To locate __libc_early_init, a direct symbol lookup function is used,
_dl_lookup_direct.  It does not search the entire symbol scope and
consults merely a single link map.  This function could also be used
to implement lookups in the vDSO (as an optimization).

A per-namespace variable (libc_map) is added for locating libc.so,
to avoid repeated traversals of the search scope.  It is similar to
GL(dl_initfirst).  An alternative would have been to thread a context
argument from _dl_open down to _dl_map_object_from_fd (where libc.so
is identified).  This could have avoided the global variable, but
the change would be larger as a result.  It would not have been
possible to use this to replace GL(dl_initfirst) because that global
variable is used to pass the function pointer past the stack switch
from dl_main to the main program.  Replacing that requires adding
a new argument to _dl_init, which in turn needs changes to the
architecture-specific libc.so startup code written in assembler.

__libc_early_init should not be used to replace _dl_var_init (as
it exists today on some architectures).  Instead, _dl_lookup_direct
should be used to look up a new variable symbol in libc.so, and
that should then be initialized from the dynamic loader, immediately
after the object has been loaded in _dl_map_object_from_fd (before
relocation is run).  This way, more IFUNC resolvers which depend on
these variables will work.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

csu/init-first.c

@@ -16,7 +16,6 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
-#include <ctype.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <fcntl.h>
@@ -75,9 +74,6 @@ _init_first (int argc, char **argv, char **envp)
 
   __init_misc (argc, argv, envp);
 
-  /* Initialize ctype data.  */
-  __ctype_init ();
-
 #if defined SHARED && !defined NO_CTORS_DTORS_SECTIONS
   __libc_global_ctors ();
 #endif

csu/libc-start.c

@@ -22,6 +22,7 @@
 #include <ldsodefs.h>
 #include <exit-thread.h>
 #include <libc-internal.h>
+#include <elf/libc-early-init.h>
 
 #include <elf/dl-tunables.h>
 
@@ -238,6 +239,10 @@ LIBC_START_MAIN (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
     __cxa_atexit ((void (*) (void *)) rtld_fini, NULL, NULL);
 
 #ifndef SHARED
+  /* Perform early initialization.  In the shared case, this function
+     is called from the dynamic loader as early as possible.  */
+  __libc_early_init ();
+
   /* Call the initializer of the libc.  This is only needed here if we
      are compiling for the static library in which case we haven't
      run the constructors in `_dl_start_user'.  */

elf/Makefile

@@ -25,7 +25,7 @@ headers		= elf.h bits/elfclass.h link.h bits/link.h
 routines	= $(all-dl-routines) dl-support dl-iteratephdr \
 		  dl-addr dl-addr-obj enbl-secure dl-profstub \
 		  dl-origin dl-libc dl-sym dl-sysdep dl-error \
-		  dl-reloc-static-pie
+		  dl-reloc-static-pie libc_early_init
 
 # The core dynamic linking functions are in libc for the static and
 # profiled libraries.
@@ -33,7 +33,8 @@ dl-routines	= $(addprefix dl-,load lookup object reloc deps hwcaps \
 				  runtime init fini debug misc \
 				  version profile tls origin scope \
 				  execstack open close trampoline \
-				  exception sort-maps)
+				  exception sort-maps lookup-direct \
+				  call-libc-early-init)
 ifeq (yes,$(use-ldconfig))
 dl-routines += dl-cache
 endif

elf/Versions

@@ -26,6 +26,7 @@ libc {
     _dl_open_hook; _dl_open_hook2;
     _dl_sym; _dl_vsym;
     __libc_dlclose; __libc_dlopen_mode; __libc_dlsym; __libc_dlvsym;
+    __libc_early_init;
 
     # Internal error handling support.  Interposes the functions in ld.so.
     _dl_signal_exception; _dl_catch_exception;

elf/dl-call-libc-early-init.c

@@ -0,0 +1,41 @@
+/* Invoke the early initialization function in libc.so.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <assert.h>
+#include <ldsodefs.h>
+#include <libc-early-init.h>
+#include <link.h>
+#include <stddef.h>
+
+void
+_dl_call_libc_early_init (struct link_map *libc_map)
+{
+  /* There is nothing to do if we did not actually load libc.so.  */
+  if (libc_map == NULL)
+    return;
+
+  const ElfW(Sym) *sym
+    = _dl_lookup_direct (libc_map, "__libc_early_init",
+                         0x069682ac, /* dl_new_hash output.  */
+                         "GLIBC_PRIVATE",
+                         0x0963cf85); /* _dl_elf_hash output.  */
+  assert (sym != NULL);
+  __typeof (__libc_early_init) *early_init
+    = DL_SYMBOL_ADDRESS (libc_map, sym);
+  early_init ();
+}

elf/dl-load.c

@@ -30,6 +30,7 @@
 #include <sys/param.h>
 #include <sys/stat.h>
 #include <sys/types.h>
+#include <gnu/lib-names.h>
 
 /* Type for the buffer we put the ELF header and hopefully the program
    header.  This buffer does not really have to be too large.  In most
@@ -1374,6 +1375,14 @@ cannot enable executable stack as shared object requires");
     add_name_to_object (l, ((const char *) D_PTR (l, l_info[DT_STRTAB])
 			    + l->l_info[DT_SONAME]->d_un.d_val));
 
+  /* If we have newly loaded libc.so, update the namespace
+     description.  */
+  if (GL(dl_ns)[nsid].libc_map == NULL
+      && l->l_info[DT_SONAME] != NULL
+      && strcmp (((const char *) D_PTR (l, l_info[DT_STRTAB])
+		  + l->l_info[DT_SONAME]->d_un.d_val), LIBC_SO) == 0)
+    GL(dl_ns)[nsid].libc_map = l;
+
   /* _dl_close can only eventually undo the module ID assignment (via
      remove_slotinfo) if this function returns a pointer to a link
      map.  Therefore, delay this step until all possibilities for

elf/dl-lookup-direct.c

@@ -0,0 +1,116 @@
+/* Look up a symbol in a single specified object.
+   Copyright (C) 1995-2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <ldsodefs.h>
+#include <string.h>
+#include <elf_machine_sym_no_match.h>
+#include <dl-hash.h>
+
+/* This function corresponds to do_lookup_x in elf/dl-lookup.c.  The
+   variant here is simplified because it requires symbol
+   versioning.  */
+static const ElfW(Sym) *
+check_match (const struct link_map *const map, const char *const undef_name,
+             const char *version, uint32_t version_hash,
+             const Elf_Symndx symidx)
+{
+  const ElfW(Sym) *symtab = (const void *) D_PTR (map, l_info[DT_SYMTAB]);
+  const ElfW(Sym) *sym = &symtab[symidx];
+
+  unsigned int stt = ELFW(ST_TYPE) (sym->st_info);
+  if (__glibc_unlikely ((sym->st_value == 0 /* No value.  */
+                         && sym->st_shndx != SHN_ABS
+                         && stt != STT_TLS)
+                        || elf_machine_sym_no_match (sym)))
+    return NULL;
+
+  /* Ignore all but STT_NOTYPE, STT_OBJECT, STT_FUNC,
+     STT_COMMON, STT_TLS, and STT_GNU_IFUNC since these are no
+     code/data definitions.  */
+#define ALLOWED_STT \
+  ((1 << STT_NOTYPE) | (1 << STT_OBJECT) | (1 << STT_FUNC) \
+   | (1 << STT_COMMON) | (1 << STT_TLS) | (1 << STT_GNU_IFUNC))
+  if (__glibc_unlikely (((1 << stt) & ALLOWED_STT) == 0))
+    return NULL;
+
+  const char *strtab = (const void *) D_PTR (map, l_info[DT_STRTAB]);
+
+  if (strcmp (strtab + sym->st_name, undef_name) != 0)
+    /* Not the symbol we are looking for.  */
+    return NULL;
+
+  ElfW(Half) ndx = map->l_versyms[symidx] & 0x7fff;
+  if (map->l_versions[ndx].hash != version_hash
+      || strcmp (map->l_versions[ndx].name, version) != 0)
+    /* It's not the version we want.  */
+    return NULL;
+
+  return sym;
+}
+
+
+/* This function corresponds to do_lookup_x in elf/dl-lookup.c.  The
+   variant here is simplified because it does not search object
+   dependencies.  It is optimized for a successful lookup.  */
+const ElfW(Sym) *
+_dl_lookup_direct (struct link_map *map,
+                   const char *undef_name, uint32_t new_hash,
+                   const char *version, uint32_t version_hash)
+{
+  const ElfW(Addr) *bitmask = map->l_gnu_bitmask;
+  if (__glibc_likely (bitmask != NULL))
+    {
+      Elf32_Word bucket = map->l_gnu_buckets[new_hash % map->l_nbuckets];
+      if (bucket != 0)
+        {
+          const Elf32_Word *hasharr = &map->l_gnu_chain_zero[bucket];
+
+          do
+            if (((*hasharr ^ new_hash) >> 1) == 0)
+              {
+                Elf_Symndx symidx = ELF_MACHINE_HASH_SYMIDX (map, hasharr);
+                const ElfW(Sym) *sym = check_match (map, undef_name,
+                                                    version, version_hash,
+                                                    symidx);
+                if (sym != NULL)
+                  return sym;
+              }
+          while ((*hasharr++ & 1u) == 0);
+        }
+    }
+  else
+    {
+      /* Fallback code for lack of GNU_HASH support.  */
+      uint32_t old_hash = _dl_elf_hash (undef_name);
+
+      /* Use the old SysV-style hash table.  Search the appropriate
+         hash bucket in this object's symbol table for a definition
+         for the same symbol name.  */
+      for (Elf_Symndx symidx = map->l_buckets[old_hash % map->l_nbuckets];
+           symidx != STN_UNDEF;
+           symidx = map->l_chain[symidx])
+        {
+          const ElfW(Sym) *sym = check_match (map, undef_name,
+                                              version, version_hash, symidx);
+          if (sym != NULL)
+            return sym;
+        }
+    }
+
+  return NULL;
+}

elf/dl-open.c

@@ -34,6 +34,7 @@
 #include <atomic.h>
 #include <libc-internal.h>
 #include <array_length.h>
+#include <libc-early-init.h>
 
 #include <dl-dst.h>
 #include <dl-prop.h>
@@ -57,6 +58,13 @@ struct dl_open_args
      (non-negative).  */
   unsigned int original_global_scope_pending_adds;
 
+  /* Set to true by dl_open_worker if libc.so was already loaded into
+     the namespace at the time dl_open_worker was called.  This is
+     used to determine whether libc.so early initialization has
+     already been done before, and whether to roll back the cached
+     libc_map value in the namespace in case of a dlopen failure.  */
+  bool libc_already_loaded;
+
   /* Original parameters to the program and the current environment.  */
   int argc;
   char **argv;
@@ -500,6 +508,11 @@ dl_open_worker (void *a)
 	args->nsid = call_map->l_ns;
     }
 
+  /* The namespace ID is now known.  Keep track of whether libc.so was
+     already loaded, to determine whether it is necessary to call the
+     early initialization routine (or clear libc_map on error).  */
+  args->libc_already_loaded = GL(dl_ns)[args->nsid].libc_map != NULL;
+
   /* Retain the old value, so that it can be restored.  */
   args->original_global_scope_pending_adds
     = GL (dl_ns)[args->nsid]._ns_global_scope_pending_adds;
@@ -734,6 +747,11 @@ dl_open_worker (void *a)
   if (relocation_in_progress)
     LIBC_PROBE (reloc_complete, 3, args->nsid, r, new);
 
+  /* If libc.so was not there before, attempt to call its early
+     initialization routine.  */
+  if (!args->libc_already_loaded)
+    _dl_call_libc_early_init (GL(dl_ns)[args->nsid].libc_map);
+
 #ifndef SHARED
   DL_STATIC_INIT (new);
 #endif
@@ -828,6 +846,8 @@ no more namespaces available for dlmopen()"));
   args.caller_dlopen = caller_dlopen;
   args.map = NULL;
   args.nsid = nsid;
+  /* args.libc_already_loaded is always assigned by dl_open_worker
+     (before any explicit/non-local returns).  */
   args.argc = argc;
   args.argv = argv;
   args.env = env;
@@ -856,6 +876,11 @@ no more namespaces available for dlmopen()"));
   /* See if an error occurred during loading.  */
   if (__glibc_unlikely (exception.errstring != NULL))
     {
+      /* Avoid keeping around a dangling reference to the libc.so link
+	 map in case it has been cached in libc_map.  */
+      if (!args.libc_already_loaded)
+	GL(dl_ns)[nsid].libc_map = NULL;
+
       /* Remove the object from memory.  It may be in an inconsistent
 	 state if relocation failed, for example.  */
       if (args.map)

elf/libc-early-init.h

@@ -0,0 +1,35 @@
+/* Early initialization of libc.so.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _LIBC_EARLY_INIT_H
+#define _LIBC_EARLY_INIT_H
+
+struct link_map;
+
+/* If LIBC_MAP is not NULL, look up the __libc_early_init symbol in it
+   and call this function.  */
+void _dl_call_libc_early_init (struct link_map *libc_map) attribute_hidden;
+
+/* In the shared case, this function is defined in libc.so and invoked
+   from ld.so (or on the fist static dlopen) after complete relocation
+   of a new loaded libc.so, but before user-defined ELF constructors
+   run.  In the static case, this function is called directly from the
+   startup code.  */
+void __libc_early_init (void);
+
+#endif /* _LIBC_EARLY_INIT_H */

elf/libc_early_init.c

@@ -0,0 +1,27 @@
+/* Early initialization of libc.so, libc.so side.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <ctype.h>
+#include <libc-early-init.h>
+
+void
+__libc_early_init (void)
+{
+  /* Initialize ctype data.  */
+  __ctype_init ();
+}

elf/rtld.c

@@ -46,6 +46,7 @@
 #include <stackinfo.h>
 #include <not-cancel.h>
 #include <array_length.h>
+#include <libc-early-init.h>
 
 #include <assert.h>
 
@@ -2372,6 +2373,9 @@ ERROR: '%s': cannot process note segment.\n", _dl_argv[0]);
       rtld_timer_accum (&relocate_time, start);
     }
 
+  /* Relocation is complete.  Perform early libc initialization.  */
+  _dl_call_libc_early_init (GL(dl_ns)[LM_ID_BASE].libc_map);
+
   /* Do any necessary cleanups for the startup OS interface code.
      We do these now so that no calls are made after rtld re-relocation
      which might be resolved to different functions than we expect.

sysdeps/generic/ldsodefs.h

@@ -336,6 +336,10 @@ struct rtld_global
        recursive dlopen calls from ELF constructors.  */
     unsigned int _ns_global_scope_pending_adds;
 
+    /* Once libc.so has been loaded into the namespace, this points to
+       its link map.  */
+    struct link_map *libc_map;
+
     /* Search table for unique objects.  */
     struct unique_sym_table
     {
@@ -946,6 +950,19 @@ extern lookup_t _dl_lookup_symbol_x (const char *undef,
      attribute_hidden;
 
 
+/* Restricted version of _dl_lookup_symbol_x.  Searches MAP (and only
+   MAP) for the symbol UNDEF_NAME, with GNU hash NEW_HASH (computed
+   with dl_new_hash), symbol version VERSION, and symbol version hash
+   VERSION_HASH (computed with _dl_elf_hash).  Returns a pointer to
+   the symbol table entry in MAP on success, or NULL on failure.  MAP
+   must have symbol versioning information, or otherwise the result is
+   undefined.  */
+const ElfW(Sym) *_dl_lookup_direct (struct link_map *map,
+				    const char *undef_name,
+				    uint32_t new_hash,
+				    const char *version,
+				    uint32_t version_hash) attribute_hidden;
+
 /* Add the new link_map NEW to the end of the namespace list.  */
 extern void _dl_add_to_namespace_list (struct link_map *new, Lmid_t nsid)
      attribute_hidden;

sysdeps/mach/hurd/i386/init-first.c

@@ -17,7 +17,6 @@
    <https://www.gnu.org/licenses/>.  */
 
 #include <assert.h>
-#include <ctype.h>
 #include <hurd.h>
 #include <stdio.h>
 #include <unistd.h>
@@ -85,9 +84,6 @@ posixland_init (int argc, char **argv, char **envp)
 #endif
   __init_misc (argc, argv, envp);
 
-  /* Initialize ctype data.  */
-  __ctype_init ();
-
 #if defined SHARED && !defined NO_CTORS_DTORS_SECTIONS
   __libc_global_ctors ();
 #endif